With the explosion in the demand for wireless Internet services, a number of competing solutions have been developed. UMTS (Universal Mobile Terrestrial Service) standardization has lead to the 3 Gpp standard which offers a 2 Mbps data rate per sector. Work is underway on HSPDA (high speed data access), a higher speed packet data access variation. IS-2000, an evolution of IS-95 provides HDR (High Speed Data Rate) and 1XEV (1X Evolution) which allow wireless Internet browsing at a rate of 7.2 Mbps per sector. Notwithstanding these solutions, there is still the demand to push rates higher.
Recently, it has been proposed to use BLAST (Bell Labs Layered Space Time) which is a layered space-time coding approach, as a wireless data solution. Referring to FIG. 1, the basic concept behind this layered space-time coding approach involves, at the transmit side, a demultiplexer 10 which demultiplexes a primary data stream 11 into M data substreams of equal rate. Each of the M data streams is then encoded and modulated separately in respective coding/modulating blocks 12 (12A, 12B, . . . , 12M) to produce respective encoded and modulated streams 13 (13A, 13B, . . . , 13M). There are M transmit antennas 14 (14A, 14B, . . . , 14M). A switch 16 periodically cycles the association between the modulated streams 13A, 13B, . . . , 13M and the antennas 14A, 14B, . . . , 14M. At the receive side, there are M antennas 18 (18A, 18B, . . . , 18M) which feed into a beamforming/spatial separation/substruction block 20 which performs a spatial beamforming/nulling (zero forcing) process to separate the individual coded streams and feeds these to respective individual decoders 22 (22A, 22B, . . . , 22M). The outputs of the decoders 22A, 22B, . . . , 22M are fed to a multiplexer 24 which multiplexes the signals to produce an output 25 which is an estimate of the primary data stream 11.
There are a number of variations on this architecture. One is to modify the receiver antenna pre-processing to carry out MMSE (minimum mean square error) beamforming rather than nulling in order to improve the wanted signal SNR (signal-to-noise ratio) at the expense of slightly increased ISI (inter-symbol interference). Both the MMSE and nulling approaches normally have the disadvantage that some sort of diversity of the receiver antenna array is necessarily sacrificed in the beamforming process. In order to overcome this problem, layering of the receiver processing can be employed such that after the strongest signal has been decoded (typically using the Viterbi MLSE (maximum likelihood sequence estimation) algorithm) it is subtracted from the received antenna signals in order to remove the strongest signal. This process is iterated down until detection of the weakest signal requires no nulling at all, and its diversity performance is therefore maximized. A disadvantage with this layered approach is the same as that with all subtractive multi-user detection schemes, that the wrong subtraction can cause error propagation.
There are several types of layered space-time coding structures, including horizontal BLAST (H-BLAST), diagonal BLAST (D-BLAST) and vertical BLAST. They have identical performance for both optimal linear and non-linear receivers, assuming error control coding is not used in such systems. For optimal linear reception (linear maximum likelihood), these structures have the same SNR performances as those with only a single transmit antenna and a single receive antenna, but do offer the advantage of improved spectral efficiency.
In order to achieve this improved spectral efficiency, in such systems it would be advantageous to have a large number of transmit and receive antennas, for example four of each. However, while this may be practical for larger wireless devices such as laptop computers, it is impractical for smaller hand-held devices because it is not possible to get the antennas far enough apart to ensure their independence. Because of this, for hand-held devices, a practical limit might be two transmit and two receive antennas. Also, another factor limiting the practical number of antennas is cost. Typically about two thirds of the cost of a base station transceiver is in the power amplifier plus antennas, and this will increase if more antennas are added. These factors make only a two by two system commercially practical.
By way of example, consider a system with M transmit and N receive antennas in a frequency non-selective, slowly fading channel. The sampled baseband-equivalent channel model is given byY=HS+ηwhere HεCN×M is the complex channel matrix with the (i,j)-th element being random fading between the i-th receive and j-th transmit antenna. ηεCN is the additive noise source and is modelled as a zero mean circularly symmetric complex Gaussian random vector with statistically independent elements, that is η˜CN(0,2η2IN). The i-th element of SεCM is the symbol transmitted at the i-th transmit antenna and that of YεCN is the symbol received at the i-th received antenna. The model is shown in FIG. 2.
That such a system has no improvement in SNR performance can be explained by noting that the data symbol sm is transmitted only by one antenna, and in case of full cancellation of other transmit antennas, the model of such a system is shown in FIG. 3. In this case there is one transmit antenna and N receive antennas. Therefore, for symbol Sm there is no coding gain.
It would be advantageous to have a layered space-time coding structure which provides the improved spectral efficiency, but which also provides improved SNR performance.